One system for accomplishing the confinement of an animal is disclosed in U.S. Pat. No. 5,170,149 entitled CONFINEMENT ARRANGEMENT FOR ANIMALS and issued to Yarnall, Sr. et al. This and other conventional systems mount a receiver on a collar. The collar delivers a shock to an animal wearing the collar when the animal is in relatively close proximity to a perimeter antenna wire. The conventional systems are unintentionally activated, however, by common and naturally occurring sources of unmodulated radio frequencies. These sources include, for example, lightning and household electrical motors such as those motors used in hair dryers and blenders.
Another exemplary conventional system is described in U.S. Pat. No. 4,898,120 entitled ANIMAL TRAINING AND RESTRAINING SYSTEM and issued to Brose. The '120 patent describes a system which includes a transmitter/receiver mounted on an animal's collar for transmitting a series of pulses. A control unit receives the transmitted pulses and provides them to an analog-to-digital (A/D) converter for converting the transmitted pules to digital form based on the amplitude of the received pulses. A central processing unit (CPU) in the control unit uses the digital signals to define the average strength of the transmitted pulses to ascertain if the animal has approached a predetermined distance from the transmitter. The system does not distinguish the transmitted signal, however, from naturally occurring and common sources of unmodulated radio frequencies.
Another exemplary conventional system is described in U.S. Pat. No. 4,967,695 entitled SYSTEM FOR CONTROLLING THE MOVEMENT OF AN ANIMAL and issued to Giunta. The '695 patent describes a confinement system which uses three separate antennas to overcome the problem of a temporary loss of signal when the animal changes direction of travel. The three separate antennas are linked to three individual measuring circuits, which are then connected to a comparator circuit that is linked to a switch circuit. The system has a comparator circuit and a large number of antennas and measuring circuits--all of which must fit inside the animal's receiver unit. These components may increase the size of the receiver unit. In addition, there is an increased drain on the battery used to power the receiver because of the increased number of electronic components that are located in the animal's collar unit. Finally, the manufacturing cost of the animal's receiver unit may be increased.
The foregoing illustrates the limitations known to exist in present confinement systems. Thus, it is apparent that it would be advantageous to provide an alternative confinement system which will account for common and naturally occurring sources of unmodulated radio frequencies and will include an animal's receiver unit that can overcome temporary loss of signal without requiring extensive additional circuitry. Accordingly, these are the principal objects of the present invention.